1. Field of the Invention
The present invention relates to a method of screening varistors, particularly those employed as a measure for absorbing surges in electronic circuits and the like.
2. Related Art
There have been proposed various conventional methods for detecting presence/absence of an internal defect in a varistor, which is a voltage-nonlinear resistor (resistor whose resistance varies nonlinearly with applied voltage). In the method disclosed in Japanese Patent Publication (kokoku) No. 6-70658, a pulse is applied to a varistor such that if an internal defect is present, the varistor is destroyed. In the method disclosed in Japanese Patent Publication (kokoku) Nos. 4-66082 and 1-13201, the current-voltage characteristic of a varistor is measured and compared with that of a normal varistor in order to detect presence/absence of an internal defect in the varistor.
The method in which a pulse is applied to a varistor in order to destroy the varistor if an internal defect is present has drawbacks in that if the energy of the pulse is excessively high, the pulse deteriorates non-defective varistors, and if the energy of the pulse is excessively low, detection of defects becomes impossible. Therefore, strict setting and control of pulse energy have been required.
The method in which the current-voltage characteristic of a varistor is measured for the detection of presence/absence of internal defect makes use of a statistical correlation between presence of an internal defect in a varistor and variation in the current-voltage characteristic of the varistor. Thus, in order to reliably reject defective varistors, the conditions for screening non-defective varistors must be made stricter in order to narrow the screening range, leading to the possibility that non-defective products may also be rejected as defectives.